The present invention relates to a method of producing a vertically inhomogeneous platinum or gold distribution in a semiconductor substrate and in a semiconductor device.
Introducing heavy metals, such as platinum (Pt) or gold (Au), into a semiconductor device may often be used to influence the velocity of the recombination of electrons and holes in the device. The characteristic time constant for this is the so-called minority charge carrier life. The heavy metals form impurities having an energy level in the area of the band center e.g. in a silicon device, and lead to an effective increase in the recombination rate both for electrons and for holes due to the high trapping cross-section connected thereto. Alternatively, the irradiation with high-energy particles, such as electrons, protons or helium ions, may also be employed for this. In power diodes, for example, this behavior is utilized to reduce their switching losses by suitable doping. Apart from the desired reduction in the switching losses, the introduction of the heavy metals, however, also leads to an increase in the forward-voltage drop Vf and the leakage current of the diode. Since platinum, in contrast to gold, has a more decentralized energetic position in the prohibitive band between the valency band and the conduction band of silicon—as exemplary semiconductor material—, and platinum-doped diodes therefore have a lower leakage current in the blocking state of the diode, platinum is often preferred for doping such diodes. However, doping with Au or other heavy metals may also be performed. However, suitable methods for producing inhomogeneous platinum distributions in a thermal process still do not exist.
Such diodes may find application for controlling inductive loads, such as electric motors in so-called voltage link converters for variable-speed drives, which are employed both in the consumer area, such as in washing machines, air conditioning devices, etc., as well as in drive engineering for railways and industry, and today rapidly switching MOS power transistors, such as IGBTs (insulated-gate bipolar transistors), are used in the higher voltage range.
These devices may be dimensioned to a blocking capability of 600V up to 6.5 kV, depending on the field of application. By alternating switch-on and off in a bridge circuit, a desired frequency may be generated by pulse width modulation at the output. So as to keep the switching losses as low as possible, a high switching speed is intended. Due to the inductive load, a high induction voltage, which may destroy the active switch, may develop upon sudden switch-off. For this reason, a free-wheeling diode, which further guides the current flow driven by the inductance, may be provided in a parallel branch. When switching the transistor on again, the current possibly still flowing through the diode is commutated to the IGBT. Here, the switch-on speed of the transistor determines the steepness of the current decrease in the diode, the so-called dI/dt(I=current, t=time).